So everybody is "at fault" yet nobody is "at fault" for doing something majorly wrong knowingly. Spanair claims they were never told about the change in SOPs recomendations, Boeing claims they told everybody to never trust the TOWS 100% and constanly check them and the pilots thought they had followed the checklist and thought they had actually set the flaps/slats. If they are proven not to have been overly distracted and to have performed the TOWS check in the previous flight from Barcelona, then it's a case of a single, "reasonable", unlucky, mistake.
Everybody thought they were doing the right thing yet none of them did 100%.
Seems that this cheese was small in size but had plenty of really tiny small holes aligned ...
I like this summation so what should/can be done?
I say you need some historical data
Examine the failure rate for the individual faults, if the actual fleet failure rates in combination predict the accident with a greater than 60% probability than look at the manufacturer for preventive action. If the failure rates in combination are much less than this than look at the operator for preventive action